[PATCH] e1000e: prevent division by zero if TIMINCA is zero

[PATCH] e1000e: prevent division by zero if TIMINCA is zero

[Denys Vlasenko]

Users report that under VMWare, er32(TIMINCA) returns zero.
This causes division by zero at init time as follows:

 ==>            incvalue = er32(TIMINCA) & E1000_TIMINCA_INCVALUE_MASK;
                for (i = 0; i < E1000_MAX_82574_SYSTIM_REREADS; i++) {
                        /* latch SYSTIMH on read of SYSTIML */
                        systim_next = (cycle_t)er32(SYSTIML);
                        systim_next |= (cycle_t)er32(SYSTIMH) << 32;

                        time_delta = systim_next - systim;
                        temp = time_delta;
 ====>                  rem = do_div(temp, incvalue);

This change makes kernel survive this, and users report that
NIC does work after this change.

Since on real hardware incvalue is never zero, this should not affect
real hardware use case.

Signed-off-by: Denys Vlasenko <dvlasenk@redhat.com>
CC: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
CC: "Ruinskiy, Dima" <dima.ruinskiy@intel.com>
CC: intel-wired-lan@lists.osuosl.org
CC: netdev@vger.kernel.org
CC: LKML <linux-kernel@vger.kernel.org>
---
 drivers/net/ethernet/intel/e1000e/netdev.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/drivers/net/ethernet/intel/e1000e/netdev.c b/drivers/net/ethernet/intel/e1000e/netdev.c
index 269087c..0626935 100644
--- a/drivers/net/ethernet/intel/e1000e/netdev.c
+++ b/drivers/net/ethernet/intel/e1000e/netdev.c
@@ -4315,7 +4315,8 @@ static cycle_t e1000e_cyclecounter_read(const struct cyclecounter *cc)
 
 			time_delta = systim_next - systim;
 			temp = time_delta;
-			rem = do_div(temp, incvalue);
+			/* VMWare users have seen incvalue of zero, don't div / 0 */
+			rem = incvalue ? do_div(temp, incvalue) : (time_delta != 0);
 
 			systim = systim_next;
 
-- 
1.8.1.4

Re: [Intel-wired-lan] [PATCH] e1000e: prevent division by zero if TIMINCA is zero

[Rustad, Mark D]

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Denys Vlasenko <dvlasenk@redhat.com> wrote:

> Users report that under VMWare, er32(TIMINCA) returns zero.
> This causes division by zero at init time as follows:
>
>  ==>            incvalue = er32(TIMINCA) & E1000_TIMINCA_INCVALUE_MASK;
>                 for (i = 0; i < E1000_MAX_82574_SYSTIM_REREADS; i++) {
>                         /* latch SYSTIMH on read of SYSTIML */
>                         systim_next = (cycle_t)er32(SYSTIML);
>                         systim_next |= (cycle_t)er32(SYSTIMH) << 32;
>
>                         time_delta = systim_next - systim;
>                         temp = time_delta;
>  ====>                  rem = do_div(temp, incvalue);
>
> This change makes kernel survive this, and users report that
> NIC does work after this change.
>
> Since on real hardware incvalue is never zero, this should not affect
> real hardware use case.
>
> Signed-off-by: Denys Vlasenko <dvlasenk@redhat.com>
> CC: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
> CC: "Ruinskiy, Dima" <dima.ruinskiy@intel.com>
> CC: intel-wired-lan@lists.osuosl.org
> CC: netdev@vger.kernel.org
> CC: LKML <linux-kernel@vger.kernel.org>
> ---
>  drivers/net/ethernet/intel/e1000e/netdev.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/net/ethernet/intel/e1000e/netdev.c  
> b/drivers/net/ethernet/intel/e1000e/netdev.c
> index 269087c..0626935 100644
> --- a/drivers/net/ethernet/intel/e1000e/netdev.c
> +++ b/drivers/net/ethernet/intel/e1000e/netdev.c
> @@ -4315,7 +4315,8 @@ static cycle_t e1000e_cyclecounter_read(const  
> struct cyclecounter *cc)
>
>  			time_delta = systim_next - systim;
>  			temp = time_delta;
> -			rem = do_div(temp, incvalue);
> +			/* VMWare users have seen incvalue of zero, don't div / 0 */
> +			rem = incvalue ? do_div(temp, incvalue) : (time_delta != 0);
>
>  			systim = systim_next;
>

I seem to recall that this was rejected before because it really is  
VMWare's bug and, if they fix it, any existing VMs that use this will just  
work. Changing the driver will only fix it for vms that install a new  
driver. I don't object to doing it, it just seems like not the most  
effective place to address the issue.

Re: [Intel-wired-lan] [PATCH] e1000e: prevent division by zero if TIMINCA is zero

[Jarod Wilson]

On Fri, May 06, 2016 at 11:43:17PM +0000, Rustad, Mark D wrote:
> Denys Vlasenko <dvlasenk@redhat.com> wrote:
> 
> >Users report that under VMWare, er32(TIMINCA) returns zero.
> >This causes division by zero at init time as follows:
> >
> > ==>            incvalue = er32(TIMINCA) & E1000_TIMINCA_INCVALUE_MASK;
> >                for (i = 0; i < E1000_MAX_82574_SYSTIM_REREADS; i++) {
> >                        /* latch SYSTIMH on read of SYSTIML */
> >                        systim_next = (cycle_t)er32(SYSTIML);
> >                        systim_next |= (cycle_t)er32(SYSTIMH) << 32;
> >
> >                        time_delta = systim_next - systim;
> >                        temp = time_delta;
> > ====>                  rem = do_div(temp, incvalue);
> >
> >This change makes kernel survive this, and users report that
> >NIC does work after this change.
> >
> >Since on real hardware incvalue is never zero, this should not affect
> >real hardware use case.
...
> I seem to recall that this was rejected before because it really is VMWare's
> bug and, if they fix it, any existing VMs that use this will just work.
> Changing the driver will only fix it for vms that install a new driver. I
> don't object to doing it, it just seems like not the most effective place to
> address the issue.

You could also have people who never update VMWare, for whom a kernel
work-around would be better. I think it'd be best to address it both at
the driver level and the emulated hardware level, to improve things for
the most possible users. Those who update neither hypervisor or
kernel/driver, well, they reap what they sow.

-- 
Jarod Wilson
jarod@redhat.com

Re: [Intel-wired-lan] [PATCH] e1000e: prevent division by zero if TIMINCA is zero

[Mark D Rustad]

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Jarod Wilson <jarod@redhat.com> wrote:

> On Fri, May 06, 2016 at 11:43:17PM +0000, Rustad, Mark D wrote:
>> Denys Vlasenko <dvlasenk@redhat.com> wrote:
>>
>>> Users report that under VMWare, er32(TIMINCA) returns zero.
>>> This causes division by zero at init time as follows:
>>>
>>> ==>            incvalue = er32(TIMINCA) & E1000_TIMINCA_INCVALUE_MASK;
>>>                for (i = 0; i < E1000_MAX_82574_SYSTIM_REREADS; i++) {
>>>                        /* latch SYSTIMH on read of SYSTIML */
>>>                        systim_next = (cycle_t)er32(SYSTIML);
>>>                        systim_next |= (cycle_t)er32(SYSTIMH) << 32;
>>>
>>>                        time_delta = systim_next - systim;
>>>                        temp = time_delta;
>>> ====>                  rem = do_div(temp, incvalue);
>>>
>>> This change makes kernel survive this, and users report that
>>> NIC does work after this change.
>>>
>>> Since on real hardware incvalue is never zero, this should not affect
>>> real hardware use case.
> ...
>> I seem to recall that this was rejected before because it really is  
>> VMWare's
>> bug and, if they fix it, any existing VMs that use this will just work.
>> Changing the driver will only fix it for vms that install a new driver. I
>> don't object to doing it, it just seems like not the most effective  
>> place to
>> address the issue.
>
> You could also have people who never update VMWare, for whom a kernel
> work-around would be better. I think it'd be best to address it both at
> the driver level and the emulated hardware level, to improve things for
> the most possible users. Those who update neither hypervisor or
> kernel/driver, well, they reap what they sow.

That is a sound argument for doing both. I would expect that there are more  
frozen VM images than host environments, but I can certainly imagine that  
some choose to freeze their host. Of course if everything is frozen there  
is no point at all. :-)

I am on an extended vacation, and don't work on e1000e anyway, so I will  
quit my kibitzing here.

--
Mark Rustad, MRustad@gmail.com

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RE: [Intel-wired-lan] [PATCH] e1000e: prevent division by zero if TIMINCA is zero

[Brown, Aaron F]

> From: Intel-wired-lan [mailto:intel-wired-lan-bounces@lists.osuosl.org] On
> Behalf Of Denys Vlasenko
> Sent: Friday, May 6, 2016 12:42 PM
> To: Kirsher, Jeffrey T <jeffrey.t.kirsher@intel.com>
> Cc: intel-wired-lan@lists.osuosl.org; Denys Vlasenko
> <dvlasenk@redhat.com>; LKML <linux-kernel@vger.kernel.org>;
> netdev@vger.kernel.org
> Subject: [Intel-wired-lan] [PATCH] e1000e: prevent division by zero if
> TIMINCA is zero
> 
> Users report that under VMWare, er32(TIMINCA) returns zero.
> This causes division by zero at init time as follows:
> 
>  ==>            incvalue = er32(TIMINCA) & E1000_TIMINCA_INCVALUE_MASK;
>                 for (i = 0; i < E1000_MAX_82574_SYSTIM_REREADS; i++) {
>                         /* latch SYSTIMH on read of SYSTIML */
>                         systim_next = (cycle_t)er32(SYSTIML);
>                         systim_next |= (cycle_t)er32(SYSTIMH) << 32;
> 
>                         time_delta = systim_next - systim;
>                         temp = time_delta;
>  ====>                  rem = do_div(temp, incvalue);
> 
> This change makes kernel survive this, and users report that
> NIC does work after this change.
> 
> Since on real hardware incvalue is never zero, this should not affect
> real hardware use case.
> 
> Signed-off-by: Denys Vlasenko <dvlasenk@redhat.com>
> CC: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
> CC: "Ruinskiy, Dima" <dima.ruinskiy@intel.com>
> CC: intel-wired-lan@lists.osuosl.org
> CC: netdev@vger.kernel.org
> CC: LKML <linux-kernel@vger.kernel.org>
> ---
>  drivers/net/ethernet/intel/e1000e/netdev.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)

As Mark Rustad pointed out I recall this was earlier rejected as something that is a VMWare error and it should be fixed there so that existing VMs will start working without installing a new driver.  Having said that, it does not seem to be causing any harm in my testing, so...

Tested-by: Aaron Brown <aaron.f.brown@intel.com>